The aims of this project were 1) to determine the acute effect of feeding fasted humans with a meal rich in animal and vegetable fat on the level and composition of postprandial lipoproteins appearing in the blood four hours after the meal, and 2) to compare the atherogenic potencies of these postprandial lipoproteins in cultured arterial wall cells. We found that 1) the fat composition postprandial chylomicrons is generally reflective of the fat composition of a meal given to the study subject but is not the same; 2) the postprandial increases of plasma triglycerides is somewhat greater with a meal high in vegetable fat than with a meal rich in animal fat, and 3) the postprandial chylomicrons produced by a meal rich in vegetable fat have greater potencies to produce foam cell formation in cultured macrophages and to injure cultured endothelial cells than those produced by a meal rich in animal fat. The findings of this study are contrary to a general concept that animal fat is more atherogenic than vegetable fat.